Prostheses comprising a portion made of deformable material are already known. In French patent No. 2 623 085 in the name of Francis Breard, there is described a kind of spacer having two ends and suitable for being inserted between the spinous processes of two adjacent vertebrae. The spacer is held by means of ligaments passing through lateral holes.
A prosthesis of a very similar design is described in European patent No. 0 322 334 to inventor Jean-Jacques Bronsard. One or more hollow resilient cylindrical pads are described therein as being interposed between the spinous processes of two adjacent vertebrae, and as being secured by means of a ligament passing through the pads. Other inter-process prostheses of a variety of shapes are described in French patents Nos. 2 717 675 and 2 775 183 to Dr. Jean Taylor.
Although those known devices provide results that are advantageous in terms of disk spacing, by being secured between spinous processes, they nevertheless suffer from drawbacks that are not negligible since they do not provide any means for recovering the ability to support loads that are appropriate to physiological requirements. The absorption of load transmission between vertebrae has until now remained partial only.
Since such prostheses interposed between the spinous processes are off-center relative to the center of gravity of the vertebrae bodies, which carry the maximum load, whereas the major fraction of the load passes via an axis situated in the centers of the vertebral bodies.
The first disadvantage of such prior devices is that only a portion of the load is absorbed by the prosthesis, thus preventing it from having a damping function that is fully effective.
The second disadvantage is that the articular mobility of prostheses of that type is small, with full control over flexing, extension, and rotation of the spine then being substantially limited.
The third disadvantage is that those known devices are all invasive since it is necessary to remove the healthy posterior ligament or to damage the adjacent lateral muscles in order to put them in place.